In general, various detection devices are proposed which detect physical amounts such as pressure and magnetism and convert them into electrical signals. Since in such a detection device, a sensor outputs a variation in the physical amount as a differential value, the detection device includes a processing circuit for converting, by integration with an integrator circuit, a detection signal from the sensor into the voltage of a waveform similar to the variation in the physical amount.
FIG. 8 shows an example of a processing circuit in a pressure detection device which detects, as a physical amount, the combustion pressure of an engine. In FIG. 8, the processing circuit 100 is formed with two operational amplifiers A1 and A2 which are an integrator circuit that integrates a detection signal and an amplifier circuit that amplifies an output signal obtained by the integration. One terminal of a piezoelectric element 101 which detects pressure is connected through a conductive member 102 via the input capacitor C1 of the processing circuit 100 to the inverting input terminal of the operational amplifier A1. The other terminal of the piezoelectric element 101 is connected through the conductive member 102 to the GNU of the processing circuit 100. In this way, a charge signal Qi from the piezoelectric element 101 is fed to the inverting input terminal of the operational amplifier A1.
On the other hand, a reference voltage Vr from a reference power supply Rg1 formed with a regulator is fed to the non-inverting input terminal of the operational amplifier A1. An external power supply Vdd is input to the reference power supply Rg1, and thus the reference power supply Rg1 outputs the stable reference voltage Vr. Between the inverting input terminal of the operational amplifier A1 and the output terminal of the operational amplifier A1, a charge capacitor C2 and a discharge resistor R1 are connected. An output signal A1out obtained by integrating the charge signal Qi to convert it into a voltage is obtained from the output terminal of the operational amplifier A1.
The output signal A1out is fed to the non-inverting input terminal of the operational amplifier A2 which is an amplifier circuit. The non-inverting input terminal is connected through a resistor R4 to the reference voltage Vr. The non-inverting input terminal of the operational amplifier A2 is connected through a resistor R5 to the reference voltage Vr and is connected through a resistor R6 to the output terminal. In this way, it is possible to obtain an amplified output signal Vout from the output terminal of the operational amplifier A2.
On the other hand, the piezoelectric element 101 is stored in the enclosure 110 of the detection device, and the enclosure 110 is grounded (earth) in common with the engine (not shown) when a combustion pressure sensor for detecting the combustion pressure of the engine is used. The reason why the inputs of the integrator circuit of the operational amplifier A1 and the amplifier circuit of the operational amplifier A2 are connected to the reference voltage Vr is that single-power supply drive (the power supply Vdd) is adopted to simplify the power supply of the processing circuit 100 and hence as a reference for operating the operational amplifiers A1 and A2, an intermediate voltage between the power supply Vdd and the GND is needed. In the illustrated case, power supply Vdd=DC+5V and reference voltage Vr=DC+1V.
The basic operation of the pressure detection device will then be described with reference to FIG. 9. FIG. 9(a) schematically shows the differentiated waveform of the charge signal Qi with respect to a time t when the piezoelectric element 101 detects a variation in the combustion pressure at a period TO. The charge signal Qi is fed through the input capacitor C1 shown in FIG. 8 to the inverting input terminal of the operational amplifier A1 in the processing circuit 100.
FIG. 9(b) shows an example of the voltage waveform of the output signal A1out output from the output terminal of the operational amplifier A1. Since the operational amplifier A1 is operated with reference to the reference voltage Vr, the charge signal Qi is converted by integration into voltage, and the output signal A1out similar to a variation in the voltage is obtained from the output terminal of the operational amplifier A1. Since the operational amplifier A1 uses the reference voltage Vr as an operation reference, for example, when the charge signal Qi is not present such as at time t1, the voltage level of the output signal A1out is essentially equal to the reference voltage Vr (in the illustrated case, DC 1V).
FIG. 9(c) shows an example of the voltage waveform of the output signal Vout output from the output terminal of the operational amplifier A2. Here, the operational amplifier A2 is operated as a non-inverting amplifier circuit which uses the reference voltage Vr as an operation reference, the output signal Vout thereof is in phase with the output signal A1out which is input and the amplitude thereof has a magnitude which is amplified at a predetermined amplification factor. Since the operational amplifier A2 also uses the reference voltage Vr as an operation reference, for example, when the output signal A1out which is input is not present as at time t1, the voltage level of the output signal Vout is essentially equal to the reference voltage Vr (in the illustrated case, DC 1V).
In this case, since the piezoelectric element 101 serving as a pressure detection element has an extremely high direct-current impedance, and the amount of charge of the charge signal Qi serving as the detection signal is low, as the operational amplifier A1 to which the charge signal Qi is fed, an operational amplifier which has performance of an extremely high input impedance is needed, and the charge capacitor C2 and the discharge resistor R1 also need to have high impedances. In this way, the input circuit of the operational amplifier A1, that is, the circuit which includes the inverting input terminal of the operational amplifier A1, the input capacitor C1, the charge capacitor C2 and the discharge resistor R1, has an impedance higher than a peripheral circuit, and has the characteristic of being easily affected by induction noise and leakage current from the outside.
If induction noise from the outside is input to the input circuit of the operational amplifier A1 in a mixed manner, a large error occurs in the output signal A1out since the noise component of operational amplifier A1 is integrated together with the charge signal Qi. If the leakage current is passed from the input circuit of the operational amplifier A1 to the power supply Vdd and the GND in the vicinity thereof, a direct-current offset voltage is produced in the output signal A1out of the operational amplifier A1, with the result that, in the worst case, there is a danger that the potential of the output signal A1out exceeds the power supply Vdd or the GND. Hence, in order to highly accurately integrate the charge signal Qi to convert it into a voltage waveform, it is significantly important that the input circuit of the operational amplifier A1 should minimize influences of induction noise and leakage current.
Under the foregoing circumstances, in a high-impedance circuit, a high-speed signal circuit and the like, it is necessary to take the measure of reducing the influences of induction noise and leakage current, and hence, various noise reduction measures are conventionally proposed.
For example, patent literature 1 discloses a semiconductor device which includes a shield means for the wiring of a multi-pin LSI. In particular, in the shield means, a guard ring is provided so as to surround a high-speed signal line which is easily affected by noise, the number of power supply terminals connected to the guard ring is set lower than the number of high-speed signal terminals and furthermore, the width of the wiring of the guard ring is changed according to the distance and route from the power supply terminal, in an attempt to improve the uniformity of the current flowing through the guard ring.
Patent literature 2 discloses a probe card which uses a multilayer substrate. In the probe card, the multilayer substrate is used to cover, for a signal line, a guard line whose potential is equal to that of the signal line both in the planar direction and the vertical direction of the multilayer substrate and furthermore, a GND line is arranged outside it. In this way, a signal which has the same potential as the signal line whose impedance is lowered by an amplifier is supplied to the guard line, and thus the occurrence of a leakage current is reduced by the guard line, with the result that it is possible to obtain a shield effect resulting from the GND line.